Effect of serum estradiol and progesterone level and progesterone/estradiol ratio on the day of human chorionic gonadotropin administration in pregnancy outcome of in vitro fertilization-embryo transplantation Chinese Journal of Tissue Engineering Research . 2010, (31): 5763 -5766 .  DOI: 10.3969/j.issn.1673-8225.2010.31.016